The production of rubber foams, natural or synthetic, by such methods as described above has been carried out for a considerable time and has now reached a well developed state. An early example is U.S. Pat. No. 2,291,213 in which rubber stock compounded from a mixture of uncured rubber, a heat-activated gas generating compound, and the usual vulcanizing ingredients is placed in a mold to substantially fill the same and heated sufficiently to liberate the gas within the rubber stock and at least partially cure the rubber while being restrained against expansion by mechanical pressure applied from e.g. A platen. The mold is then opened to permit removal and expansion of the semi-cured stock after which vulcanization is completed as usual, with or without additional expansion, giving a closed cell rubber foam.
It is desirable to produce similar expanded foam products from synthetic thermoplastic polymers such as polyolefin polymers and especially polymers and co-polymers of ethylene and vinyl acetate but the processing of such synthetic polymers has proved significantly more difficult than for rubber because they tend to be much softer than rubber and thus become more fluid when heated. Various proposals have, nonetheless, been made in the art for producing expanded vinyl foams. In U.S. Pat. No. 2,525,965, a thermoplastic resin such as a vinyl resin plastisol admixed with finely divided blowing agent is heated in a mold which it fills substantially completely under a starting low pressure, e.g. about 800 pounds per square inch of mold area, to a temperature to effect sufficient decomposition of the blowing agent as to cause up to 15% by volume of the mold charge to escape between the mold parts and form an irregular mass, after which the mold pressure was increased enough to prevent further escape, e.g. to about 4,000 p.s.i., and heating continued to complete decomposition of the blowing agent. After cooling, mold pressure was released and the molded product removed and heated, e.g. in hot water, to expand its volume.
According to U.S. Pat. No. 3,098,832, polyolefin resins blended with a heat-decomposable solid blowing agent and a heat-initiated organic peroxide cross-linking agent for the resin is placed within a confined zone in an amount occupying the entire space thereof, the zone preventing expansion, and heated to a temperature sufficient to effect at least partial cross-linking of the resin and decomposition of the blowing agent with entrapment of the gases released therefrom, the treated material being then cooled below about 150.degree. F., removed from the confined zone and heated in the range of 260.degree.-350.degree. F. or above its softening point to expand the entrapped gases. Alternatively, cross-linking can be carried out in a separate radiation step.
In some known processes, the molding composition is allowed to undergo expansion as the blowing agent is decomposed and in U.S. Pat. No. 3,812,225, partial expansion within the mold is carried out. Here, a polyolefin resin homogeneously mixed with an organic peroxide cross-linking agent and a heat-decomposable blowing agent is molded into a slab-like pre-form without decomposing the blowing agent and the pre-form is subjected in a confined zone to an external pressure by an inert gas, such as nitrogen or a platen of about 5-50 kg/cm.sup.2 and low enough to permit partial expansion thereof in the order of 5-30% of the ultimate expansion while being heated to decomposition temperature to initiate cross-linking. The heating temperature is then reduced while pressure is maintained, then such pressure is reduced to a lower level while the temperature is increased above the softening point of the material to permit expansion to reach its ultimate level.
Multi-stage expansion during a foaming process for a polyolefin resin is also shown in U.S. Pat. No. 4,671,910 wherein the basic molding composition, containing usually 10-25 parts per 100 pt resin is heated in a closed mold to selectively initiate the cross-linking reaction while substantially avoiding any decomposition of the blowing agent, then placed between two spaced-apart metal plates and heated to produce partial decomposition of the blowing agent with consequential lateral expansion between the fixed plates, after which the partially expanded product is placed under atmospheric pressure and heated to complete the decomposition of the blowing agent with free expansion thereof in all directions.
Utilization of an expandable mold for processing such an expandable composition is shown in U.S. Pat. No. 3,818,086. Here, a heat expandable olefin resin composition is placed in a gas tight mold to fill the same to 95-100% of its capacity, and heated to cause blowing agent decomposition and peroxide cross-linking under an initial pressure which is immediately released to increase mold volune 3-35 times within not more than 20 seconds and permit expansion of the composition, after which the expanded molding is cooled and removed.
In-situ expansion of the molding composition within the mold is likewise taught in U.S. Pat. No. 4,559,190 wherein polyethylene or similar resin is homogeneously mixed while molten with 0.1-2% by wt of a cross-linking agent 2-25% by wt of a blowing agent, each having a decomposition temperature higher than the polymer melt temperature, the homogeneous mixture is extruded or molded into a slab-like pre-form and the pre-form is placed within an over-sized air-tight closed container. The container is pressurize with an inert gas to a pressure of 40-70 bar (equal to about 600-1,000 p.s.i.) and heated above the decomposition temperature of the cross-linking and blowing agents for 20-45 minutes with not more than 5% expansion of the preform. The container is then depressurized for a short time with free expansion of the molded product to a volume smaller than the oversized container.
In processes of this type, expansion of the product after heat treatment within a confined mold to effect decomposition of the blowing agent, can also occur spontaneously when the mold is opened and pressure on the product is released, generally as a result of an increased level of blowing agent. Thus in U.S. Pat. No. 4,338,271 an ethylene-vinyl acetate copolymer containing greatly increased amounts of blowing or foaming agent in the order of 20-80 parts per 100 parts of copolymer plus 1-5 parts cross-linking agent after heating in a mold, which it fills to substantially full capacity, under pressure or around 150 kc/cm.sup.2 (2130 p.s.i.), and temperature above the decomposition temperature of the blowing agent will, after cooling and opening of the mold, undergo spontaneous expansion to an extent 15-50 times mold volume, thereby "popping out" of the mold. Expandable rubber compositions behave similarly as shown in U.S. Pat. No. 4,596,684 where natural or synthetic rubber, optionally containing 10-60 parts by weight of a thermoplastic resin, mixed with 10-80 parts of foaming agent per 100 parts rubber and 0.5-15 percent by cross-linking agent when filled to substantial capacity of the mold and heated to its foaming temperature under pressure in the range of 100-200 kg/cm.sup.2 (1422-2845 p.s.i.) will when the mold is opened, "abruptly pop out" of the mold, while undergoing large expansion of at least 15 times mold volume.
Despite efforts in the art as illustrated by the above disclosures, the production of high-quality expanded polyolefin resin foams with desirable uniform fine closed cell porous structure in an efficient fashion with minimal waste has been difficult to achieve in practice, and remains a desirable goal in the field. Control of the size of the charge to the mold is one important factor. Exact correspondence between the charge volume and the mold volume is virtually out of the question, especially where the charge is in the form of a pre-shaped solid slab or pre-form. If the mold is underfilled, clearance spaces are present which either serve to collect expansion gas when the same is generated and cause surface defects or permit localized expansion of the contiguous mold material resulting in the creation of large size gas bubbles in these regions. On the other hand, if the mold is overfilled and the excess charge is allowed to squeeze out after the mold is closed and decomposition of the blowing agent and release of the blowing gas has begun, this relieves the excess pressure in the regions proximate to the edges of the mold with differential expansion of the gas bubbles there. Also, cross-linking of the resin tends to occur first adjacent the outside surfaces of the charge, the heat from the heated mold naturally penetrating from the outside to the inside of the charge. Hence, cross-linking occurs preferentially at the outside of the charge and the interior material is necessarily the last material to undergo cross-linking. The melt temperature of the resin must be below the cross-linking initiation temperature and the resin thus melts before undergoing cross-linking, with the interior material being the last to melt. As gas generation and cross-linking proceed, the gas bubbles tend to migrate into the regions which remain fluid, again yielding non-uniform bubbles in the expanded foam. Moreover, if the mold is allowed to open even slightly after gas generation has begun, a delicate control is required to avoid unmanageable escape or spewing of the molten charge from the mold opening.